An apparatus is described in EP-A-257719. The apparatus disclosed in this publication consists of a submerged super heater module, consisting of a shell-tube heat exchanger, wherein the partially cooled gas is fed to the shell side of the super heater module and the steam to the tube side of the super heater module. The two flows are contacted in the super heater in a co-current mode of operation.
Applicants found that when the apparatus according to EP-A-257719 is used to cool gas comprising contaminants such as carbon, ash and/or sulphur, which is for example the case for synthesis gas produced by gasification of a gaseous or liquid hydrocarbonaceous feedstock, leakage can occur. It is believed that fouling of the apparatus at the gas side causes leakage. Although the apparatus was cleaned regularly the leakage problems persisted. Fouling, especially when the synthesis gas is produced by gasification of a liquid hydrocarbon, in particular heavy oil residues, will also result in that the heat exchange capacity of the apparatus will gradually decrease with run time. As a result, the temperature of the process gas leaving the heat exchanger will increase gradually with runtime. If the temperature of the process gas leaving the primary heat exchanger apparatus exceeds a certain temperature, typically 400-450° C., the temperature of the tubes that transmit the process gas downstream of the primary heat exchanger will be so high that they may be damaged. Therefore, the apparatus has to be shut down in order to clean the tubes. The runtime of an apparatus after which the tubes have to be cleaned is referred to as ‘cycle time’.